A preferred optical DRAW medium has a trilayer construction comprising a reflecting layer on an optically-flat substrate, an optical spacer layer overlying the reflecting layer, and a light-absorbing layer overlying the optical spacer layer. If the substrate itself is reflective, it may serve as the reflecting layer. A trilayer optical DRAW medium is schematically illustrated at FIG. 7 on page 26 of "Optical Disk Systems Emerge", by Bartolini et al., IEEE Spectrum, Vol. 15, No. 8, August 1978, pages 20-28. Its layers and their thicknesses are chosen to minimize wasteful reflections and to maximize the absorption of the focused light beam at the light-absorbing layer which in the illustration is titanium but may instead be an organic dye.
U.S. Pat. No. 4,216,501 (Bell) says that the light-absorbing layer should be selected for high absorptivity and long-term stability, suggesting materials such as titanium or rhodium. It suggests that the optical spacer layer should be selected both for transparency and for protecting the reflective surface from oxidation, and should be a dielectric material such as silicon dioxide.
U.S. Pat. No. 4,285,056 (Bell) suggests that if the material of the optical spacer layer melts at a temperature at least 300.degree. C. less than the melting point of the material of the light-absorbing layer, a trilayer optical DRAW medium would provide a much higher signal-to-noise ratio. Two mechanisms are illustrated. In one, heat from the focused, modulated light beam is said to cause the optical spacer material to decompose, sublime or otherwise experience rapidly increasing vapor pressure, and the pressure of the trapped gases can cause a bubble to form in the light-absorbing layer. In the other mechanism, the optical spacer material is said to melt along with the material of the light-absorbing layer, thus causing the material in both layers to draw up around the edge of the hole or pit created. Magnesium fluoride and lead fluoride are said to be useful for the optical spacer layer.
U.S. Pat. No. 4,300,227 (Bell) concerns a trilayer optical DRAW medium having a high-melting light-absorbing layer such as titanium and an optical spacer layer which is an organic material that melts at least 300.degree. C. less than the light-absorbing layer material. This is said to lead to a factor of about two or more reduction in the threshold recording power as compared to a medium having an inorganic optical spacer such as silicon dioxide or magnesium fluoride.
Although the trilayer optical DRAW media discussed above are effective for use with argon laser and helium/neon laser recording systems, they are not as satisfactory for recording systems based on the laser diode which is the currently preferred light source, being much less expensive, having low power requirements, and providing very fast modulation speeds. Laser diodes typically emit within the range of 700 to 900 nm.
Whatever the laser and whether or not the discontinuities are pits or bubbles, the ultimate objective is to create features such that when the recorded information is played back after prolonged storage, the reproduced signals are essentially identical to the recorded signals, as evidenced by high signal-to-noise or carrier-to-noise (C/N) ratios.
A trilayer optical DRAW medium may have additional layers such as an adhesion-promoting subbing layer or a transparent protective surface layer, but because only three layers are involved in creating pits or bubbles, the medium is nevertheless called "trilayer".